My Son Is 5'9" And Has Always Been Very Slim.

My son is 5'9" and has always been very slim. He started taking Carbamazapine in late spring/early summer for his seizures, and weighed 130# at that time. His weight has fluctuated between 128#-135# over the year prior to that. But, now, he weighs 145#, so he gained 15# in about 5 months. Can his medication cause a weight gain like that over such a short period of time? This is very unusual for him.